update code to reflect the new minimum python version 3.7

- update string format method
- remove redundant default arguments from function  calls
- remove unused imports
- remove unicode identifier from strings, it's default in py3 (see: https://stackoverflow.com/a/4182635/12201331)

allianceauth/permissions_tool/migrations/0001_initial.py

@@ -1,5 +1,4 @@
 # Generated by Django 1.10.5 on 2017-02-06 08:58
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 

allianceauth/permissions_tool/views.py

@@ -43,7 +43,7 @@ def permissions_overview(request):
 @login_required
 @permission_required('permissions_tool.audit_permissions')
 def permissions_audit(request, app_label, model, codename):
-    logger.debug("permissions_audit called by user {} on {}:{}:{}".format(request.user, app_label, model, codename))
+    logger.debug(f"permissions_audit called by user {request.user} on {app_label}:{model}:{codename}")
     try:
         perm = Permission.objects\
             .prefetch_related('group_set', 'user_set', 'state_set',